260 DK. A. GILLIGAX OX THE PETEO GRAPH Y OF [vol. IxXV 



to the outcrop of Millstone Grit of that area, are beautifully 

 rounded, resembling, indeed, in this respect, those of the Penrith 

 Sandstone of the same district. It may, therefore, be that the 

 rounded grains in both the sandstone mentioned above and in the 

 Millstone Grit have been derived from sand-dunes which fringed 

 the coast of the Carboniferous sea, but I am inclined to believe 

 that they were derived from beds of pre- Carboniferous age, for 

 reasons which will be brought forward later. 



The grains vary greatly in size, the larger showing the higher 

 degree of rounding, as (of course) is usual. Nothing will be 

 gained by describing in detail the size of the grains ; suffice it to 

 say that the larger grains are found in the Kinderscout Grit and 

 Rough Rock, while, as a rule, the Middle Grits are much finer. 

 When examined microscopically, most of the quartz shows (like 

 the larger pebbles) that it has been derived from rocks which 

 have been subjected to mechanical deformation, in that undulose 

 extinction, with crenulate and my Ionized structure, is common. 

 The larger grains show these features better than the smaller. The 

 following percentages have been obtained, by taking typical sections 

 of the grit of varying degrees of coarseness : — 



Per cent. 

 Coarse beds : Grains showing signs of pressure 84 

 Medium beds : Do. do. do. do. 62 



Fine beds : Do. do. do. do. 45 



The reasonable deduction is that most of the quartz was derived 

 from such rocks as granitoid gneisses and schists. 



Inclusions in the Quartz- Grains. 



It may be stated, first of all, that no single grain of quartz 

 entirely free of inclusions has been detected in any of the sections 

 of the grit examined : it being most usual, indeed, to find them 

 crowded with inclusions. The inclusions are of many different 

 kinds, and comprise the following : — 



(a) minerals showing good crystal form ; (6) acicular crystals possibly of 

 rutile ; (c) glass, gas, and fluid cavities ; and (d) black dust quite indeter- 

 minable, and occurring both irregularly and in rows and streams. 



The researches of Dr. W. Mackie on the quartz of granites, 

 gneisses, and schists of Scotland have shown that the type of rock 

 from which the grains have been derived iurj be deduced from an 

 examination of these inclusions. 1 He has classified the inclusions 

 as follows : — 



I. Regular (these would include those under (a) above). 

 II. Acicular ( Do. do. do. do. do. (b) do. ). 



III. Irregular ( Do. do. do. do. do. (c) and some (d) above). 



IV. Negative (that is, without inclusions, or where inclusions of the 



irregular type are so small as practically to escape notice). 



1 Trans. Edin. Geol. Soc. vol. vii (1897) pp. 148-72. 



